There has been known an image sensor for reading an image on an original document, which is used for a facsimile device, for example. The image sensor includes a plurality of light sensing elements, which are linearly arrayed. Each light sensing element consists of a photo diode and a blocking diode being connected in a back-to-back fashion.
As shown in FIGS. 16 and 17 which are a plan view and a sectional view showing a construction of the light sensing element in the image sensor, to form a photo diode PD and a blocking diode BD, the following layers are successively layered and patterned on a substrate 1 made of glass, for example; a metal electrode 2 made of chromium (Cr), for example, an ohmic contact layer 3 as an n.sup.+ layer, a photoconductive layer 4 made of hydrogenated amorphous silicon (a--Si, H), for example, a transparent electrode 5 made of indium-tin-oxide (ITO), for example, and an insulating layer 6 made of polyimide, for example. Lead wires 7a and 7b made of aluminum (Al), for example, are led from the diodes through contact holes 8a and 8b formed in the insulating layer 6.
An light receiving area A (shaded in the drawing), which receives light from above, is formed on the photo diode PD. The blocking diode BD is shielded from light by the lead wire 7b.
A plurality (n number) of light sensing elements thus constructed are disposed so as to form an array. The lead wires 7b led from each blocking diode BD is connected to a shift register SR as shown in an equivalent circuit of FIG. 18. The lead wire 7a led from each photo diode PD is grounded through a loading resistor R. An output terminal Tout is formed in the portion of the loading resistor R, which is closer to the photo diode.
The operation of reading charge out of the image sensor thus constructed will be described.
The photo diodes PD are scanned with the shift register SR. Through the scanning operation, a signal is applied to the photo diodes PD through the blocking diodes BD associated therewith, so that charge is stored into the photo diodes PD in successive manner.
During one cycle of the scanning operation, the photo diodes PD are exposed to light, so that the quantity of charge, which depends on the quantity of light received, is discharged from each photo diode. A reset signal (readout pulse signal) is successively applied to the diode series by the shift register. As a result, the quantity of charge corresponding to the quantity of discharge charge is stored again into the photo diodes.
At the time of recharge-up, current flows through a loading resistor R. The current is read out at the output terminal Tout, the form of a readout signal (Japanese Patent Laid-Open Publication No. Sho-58-62978).
The image sensor thus constructed and a method of reading out data from the image sensor suffers from noise problem for the following reason. The charge, which is caused to be discharged by the current flowing through the photo diode PD when a photo diode PD receives light, is distributed into the capacitor of the photo diode PD and the capacitor of the blocking diode BD. At this time, current flows through the loading resistor R for compensating for the reduction of the charge in the photo diode PD. This current causes noise.
To solve the problem, there is an image sensor constructed as below. A light sensing element includes a first photo diode PD not light-shielded and a second photo diode PD also not light shielded are closely disposed, and connected in a back-to-back fashion. A plurality of light sensing elements thus connected are linearly arrayed. The first and second photo diodes PD are designed such that the currents flowing through the diodes when these diodes are exposed to light, are equal to each other, whereby no current is leaked from the light sensing element.
Thus, in the image sensor, the two photo diodes (one of the diodes serves as a photo diode PD, and the other as a blocking diode BD) are connected in a back-to-back fashion. Aperture areas are formed in both the diodes, so that the currents flowing through the diodes when the diodes are exposed to light are equal to each other. As a result, current is leaked from the light sensing element, and hence the cause for generating noise is removed.
In the image sensor thus constructed, the current flowing through the loading resistor R at the time of recharge-up is read and it is used as an image signal. The circuit construction to read the current is more complicated than that to read the voltage. In a case where the charge of the light sensing elements in the image sensor is read out for each block, if the current read circuit is used, the circuit construction of it is more complicated. The image sensor of the current read type reads the current instantaneously. Accordingly, it is susceptible for noise. In other words, it is easily influenced by noise.